Chemistry Reference
In-Depth Information
Tab. 1.4
Polyethylene properties obtained with [Ni/Ph
3
P-BC/R
3
PX] catalysts
In situ catalyst:
Ni(COD)
2
Ph
3
P-BC
T
p
[
C]
[dL g
-1
]
[g cm
-3
]
Ph
3
PO
65-85
9.60
0.968
(PhO)
3
PO
60-100
5.85
0.960
Me
3
PO
75-80
4.29
0.968
(Me
2
N)
3
PO
90
2.43
0.972
(MeO)
3
PO
90-100
1.40
0.969
Abbreviations used:
in situ
catalyst: 2 mmol of each component preformed for 1 h at 50
C;
Ni(COD)
2
: bis(cyclooctadiene)nickel(0); Ph
3
P-BC: 1/1 adduct of triphenylphosphane and
p
-benzochi-
none; Me: methyl; Ph: phenyl;
T
p
: polymerization temperature;
T
m
: melt temperature (DSC);
:in-
trinsic viscosity, measured in tetraline at 140
C;
: PE density. Polymerization conditions:
in situ
cata-
lyst in 50 mL toluene, solvent: cyclohexane, ethylene pressure: 100 bar.
terminated high molecular weight PE chains. High density polyethylenes are
usually unbranched and characterized by a density
=0.96-0.97 g cm
-3
.
An outstanding property is the systems tolerance towards solvents of different
polarity with respect to maintaining catalyst activity, while at the same time creat-
ing an additional tool to control molecular weight. Increasing polarity results in
reduced molecular weight. Thus, nickel catalysts with ylide ligands have for the
first time opened access to all molecular weight ranges up to 10
6
g mol
-1
and
above; from ethene oligomers to polymers - from liquid
-olefins to soft and hard
waxes and further up to HDPE and even ultrahigh molecular weight polyethylene
(UHMW PE) with DSC melt temperatures up to 137
C.
1.3.3
Linear and Branched Macromolecules
[14]
“Tailor-made materials” require access to linear as well as to branched macro-
molecular structures. FTIR spectra of the polymer and GC analysis of the oligo-
mers in solution (if present) can be used for characterization and as a reference
for structural changes.
1.3.3.1
Linear Macromolecules
The selectivity of the bis(ylide)nickel catalysts frequently favors the formation of
linear macromolecules with unsaturated end groups. In this case the FTIR spec-
trum of a PE film of defined thickness shows almost exclusively methyl and vinyl
end groups. Their presence in equal quantities proves linearity. An example for
the catalytically controlled formation of linear, unbranched macromolecules is giv-
en in Eq. (12).
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